In a solemn ceremony Monday, President Donald Trump and Defense Secretary Jim Mattis honored those killed in the Sept. 11, 2001, attack on the Pentagon and pledged to continue the 16-year-old battle against the "maniacs" and "horrible enemies" who still threaten the nation.

"It was the worst attack on our country since Pearl Harbor and even worse because this was an attack on civilians -- innocent men, women and children whose lives were taken so needlessly" on Sept. 11, 2001, in lower Manhattan; on a field in Shanksville, Pennsylvania; and at the Pentagon, Trump said.

"The living, breathing soul of America wept with grief for every life taken on that day," Trump said on the Pentagon grounds near the memorial for the 184 who were killed when American Airlines Flight 77 slammed into the building, sending burning jet fuel down its corridors.

"We shed our tears in their memory, pledged our devotion in their honor, and turned our sorrow into an unstoppable resolve to achieve justice in their name," he said in echoing the theme of American commitment to the fight that Mattis had expressed in his previous remarks.

"The terrorists who attacked us thought they could incite fear and weaken our spirit. But America cannot be intimidated, and those who try will soon join the long list of vanquished enemies who dared to test our mettle," Trump said.

"American forces are relentlessly pursuing and destroying the enemies of all civilized people, ensuring -- and these are horrible, horrible enemies -- enemies like we've never seen before," he said, "but we're ensuring they never again have a safe haven to launch attacks against our country."

Trump never used the term "radical Islamic terrorists" that became a catchphrase in the presidential campaign, and neither did Mattis, who stressed the resiliency of the U.S. military.

"We Americans are not made of cotton candy. We are not seaweed drifting in the current. We are not intimidated by our enemies," Mattis said. "And Mr. President, your military does not scare" when confronted by "maniacs disguised in false religious garb."

"In the aftermath of the attack," the SecDef said, "our service members -- our nation -- rallied together as one. For while we had never asked for this fight, we are steadfastly committed to seeing it through, as President Trump has made abundantly clear."

The tone for the ceremony was set by Air Force Chaplain Maj. Gen. Dondi Costin in his opening prayer.

"Whereas Moses was commanded from a burning bush on holy ground to rescue his countrymen from their oppressors, our generation was commissioned from a burning building on this site to do the same," Costin said.

He prayed for Americans to be steadfast in their "righteous actions" against terrorists, and said all would be judged "in the quality of our vigilance in keeping bad guys on the run."

Earlier, family members gathered at the outdoor memorial, which consists of 184 illuminated benches for each of the victims, arranged according to the victim's age -- from three-year-old Dana Falkenberg to 71-year-old John Yamnicky Sr.

The inscription on the National 9/11 Pentagon Memorial says those who perished at the Pentagon "were mothers and fathers, husbands and wives, sisters and brothers, daughters and sons."

"They came from all walks of life: administrative assistants, doctors, educators, flight crew members, military leaders, scientists, and students," it states. "They came from towns and cities, large and small, across the United States and around the world. The youngest was only three years old; the oldest, 71."

Later at the White House, Trump signed a one-year renewal of the National Emergencies Act, to continue the national emergency declared on Sept. 14, 2001, "in response to certain terrorist acts" that occurred on 9/11.